p7-1370 Boot hang after Windows load starts, power cycle

Machine is P7-1370 (desktop), 19-months old.Windows 7 Home Premium, came with the machine.No hardware modifications.Voltages and temperatures are OK.

When I shut down, i.e. power off, and re-boot, the machine starts loading Windows, and displays "Starting Windows".Sometimes it hangs at that point. Other times, it moves onward and hangs in a black screen.

Sometimes I enter CTRL-ALT-DEL a few times, which sometimes gets booting to continue.Sometimes power cycle, which sometimes works.Sometimes I do nothing, and the machine shuts itself off!When I re-boot, Windows knows it had stopped abnormally.Sometimes I re-boot "normal windows", other times the non-normal choice (I forget what it's called).